1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a computer program, and in particular to the same suitable for detecting alteration of a printed matter.
2. Description of the Related Art
In order to detect alteration of an original document of printing, a conventional technique has been discussed that provides a printing output of a delivery document with additional information about an original document. Japanese Patent Application Laid-Open No. 2003-264685 discusses a technique which adds (embeds) an image feature quantity of a document image of an original document to a delivery document and performs printing and outputting.
In Japanese Patent Application Laid-Open No. 2003-264685, alteration of the delivery document is detected by calculating an image feature quantity from the delivery document. The calculated image feature quantity is compared with the image feature quantity added to the delivery document, and the alteration of the original document can be detected based on a comparison result.
Japanese Patent Application Laid-Open No. 2004-13287 discusses a technique that obtains character code information from an original document by an optical character reader (OCR) function, adds to a delivery document information including an authentication value calculated by a predetermined operation from the character code information and layout information, and performs printing and outputting.
In Japanese Patent Application Laid-Open No. 2004-13287, alteration of the original document is detected by obtaining the information added to the delivery document which is optically read. Subsequently, the layout information is acquired from the obtained information, and character string information is obtained from the delivery document by the OCR function according to the acquired layout information. The authentication value calculated form the predetermined operation is compared with an original authentication value included in the information added to the delivery document, and the alteration of the original document can be detected based on a comparison result. Further, the information described above is stored in an authentication agency when the delivery document is printed and output. When the alteration of the original document is be detected, the information is inquired to the authentication agency and the alteration of the original document can be detected by comparing the original authentication value included in the inquired information with the authentication value calculated by the predetermined operation.
However, the conventional techniques described above include situations as below.
According to Japanese Patent Application Laid-Open No. 2003-264685, an image is divided into blocks and an image feature quantity is defined by any of (1) a frequency spectrum, (2) a value of a result of performing arbitrary filtering processing, and (3) a ratio of white pixels and black pixels in a block image, of each block. However, any of image feature quantities described above includes too less information for authenticating a document or the information having too large size. Thus, it may be difficult to accurately detect the alteration of the document, or perform printing of the delivery document by adding the image feature quantity. Further, when dusts or stains are adhered onto the delivery document, the image feature quantity is changed and it may be difficult to accurately detect the alteration of the document.
In Japanese Patent Application Laid-Open No. 2004-13287, if the document is not altered, a reading result by the OCR when the delivery document is printed may be different from a reading result by the OCR when authentication is performed due to a situation of a reading accuracy of the OCR. Accordingly, it may be difficult to accurately detect the alteration of the document.